1. Field of the Invention
The present invention relates to a high-output power laser oscillator for use in a laser cutting machine for cutting metals or non-metals. More particularly, the invention relates to a laser oscillator which generates a circularly polarized laser beam.
2. Description of the Related Art
It is known in the art that, when cutting with a laser beam, a circularly polarized beam is preferable, and conventionally, to provide the circularly polarized beam, a linearly polarized beam generated directly from a laser oscillator is converted to a circularly polarized beam with the use of an external optical unit.
Such a conventional arrangement is shown in FIG. 9, in which only the optical unit is illustrated and conventionally known laser exciting components, such as a discharge tube, are omitted. The arrangement of FIG. 9 includes a rear mirror 1, folding mirrors 2 and 3, and an output-coupling mirror 4, all of which constitute a laser resonator. A laser beam 9 from the resonator exhibits a linear polarization having an electric vector direction orthogonal to a plane determined by three optical axes 7a, 7b and 7c. In FIG. 9, this plane is rotated by .pi./4 about the optical axis 7a. The beam 9 from the resonator has a linear polarization and a polarization direction thereof is inclined by .pi./4 with respect to a horizontal plane. This beam 9 is reflected by phase retarders 5 and 6, whereby the beam 9 is given a circular polarization.
The above-described conventional arrangement is disadvantageous in the following respects.
First, a large number of optical elements are required, which increases cost. Secondly, the system is large in size and is complicated. Finally the arrangement of the external optical system must be as precise as that of the laser resonator. In any other case, the directional stability of the laser beam will be lowered, and accordingly, the cutting characteristics degraded.